your tv probably doesnt support 60hz.
also its worth noting that not all tv support scart at 60hz.
make sure you have your xbox's output set to 60hz on the xbox dashboard.
taken from AV forums:
2.7 How do I play games requiring a 60Hz display?
Some games, regardless of whether they were bought at retail, downloaded from the Marketplace, downloaded from
Xbox Live Arcade and no matter whether they were originally intended for US/Japanese systems will only run on a console set to output a 60Hz signal. Retail games will have a label on them stating a 60Hz display is required to play.
In order to make sure 60Hz is enabled on your 360, follow these steps:
RGB SCART/Composite/S-Video connections:- In the Xbox Dashboard, select System blade.
- Select Console Settings, and then press the A button.
- Select Display, and then press the A button.
- Select PAL settings, and then press the A button.
- Select PAL-60, and then press the A button.
- When you are prompted to keep these new settings, confirm the settings, and then press the A button.